Rebar Detailing (Offset Bars)

[4wilmar]

Many detailers use offset bars (slightly bent on a 1:6 slope) to facilitate placement of lap splices. My common sense tells me they should not be allowed in areas of maximum tensile stress, but unless I have missed something, ACI does not seem to clarify where they can or cannot be used. Can anyone advise on this issue?

 

[JedClampett]

ACI 318-02 R12.14 (Commentary) says that "Splices should, if possible, be located away from points of maximum stress...."

 

[JAE]

For years, the typical ACI detailing manual, and most engineer's, specified the 1:6 taper in columns as starting just beneath the floor and tapering up through the floor thickness such that as the bars came out of the top surface of the floor structure, they would be fully offset to lap with the bars for the column above, which were set in the corners.

ACI includes in their detailing manual an alternative method where the offset occurs in an upside-down position.

Here's an attempt to describe it:
1. The lower column vertical bars are in the corners, extending straight up, and through the floor above.
2. These bars then extend from the upper floor a distance of the column lap length.
3. The upper column bars are offset to lay just inside the straight bars from below.
4. Once the upper bars lap past the straight bars, they begin to taper 1:6 outward to the four corners.
5. At the end of the taper, they then extend straight up through the floor above and the process repeats

This creates the greatest "d" value in the column reinforcing where its needed most - at the column-floor interface.

The old way created a diminished "d" at the floor, where usually the moment is greatest. Also with this "old" way, the taper occurs within the column/beam intersection and can cause a lot of interference and congestion while the alternative method only has straight, vertical bars.

I've used this detail on a lot of projects and it works well. You just have to be sure to sort of red flag this for the contractor as most will try to still put them in the "old" / wrong way.

 

[dik]

Just a reminder, the detail also requires ties to accommodate the horizontal component of the load.

Dik

 

[4wilmar]

JAE
I see a detail # 4 in ACI 315-23 with the offset bars in the "upside down" position which provides full moment capacity and makes good sense as it places the "offset" further away from the column/slab or column/beam connections and closer to an inflection point (assuming you are dealing with a rigid frame structure). I assume the other details are for structures with shear walls (no lateral loads on columns).

However what about using offset bars in walls where there are no ties (see Dik's comment) or in beams to avoid rebar congestion.

 

[JAE]

If your wall has moment at its base, then the upside-down method would also add to the "d" of the wall steel. However, most walls aren't designed for that sort of dependence on bending.

 

[UcfSE]

You can find your "d" of the steel at the splice where you have a taper. Using the reduced "d" you can go back and check the wall. If it works you're good. If it doesn't you can do a little more math and find the locations on your wall where the reduced "d" is acceptable and show those on your plans.

 

[apsix]

Am I missing something?
With walls the lapping bars can be placed side by side in the longitudinal direction, unless they are at very close spacing.

 

[4wilmar]

The rebar spacing is indeed very close in many places. Orienting the bar laps side by side (rather than front & back) also reduces the areas where we can later drill to install expansion anchors for fixing exterior stone work, so many walls have been detailed with offset bar laps. The offsets are generally placed about 1 meter above the floors, which should put them near an inflection point in a shear wall type structure (assuming the exterior walls carry the horiz. wind loads to the floor slabs in continuous beam action), so I think the offsets in the walls will be ok.

The main problem lies in the column / beam junctions where rebar congestion is very heavy. As a consequence we have reduced our max. aggregate size to 1/2" in many pours. In fact, we are using 1/2" aggregate in all columns, beams and elevated slabs. The Eng. has also aurthorized the use of offset bars in the beam steel in order to feed the horizontal bars through the column cages, but I don't feel comfortable with having any offset bars near a column/beam connection, especially in a severe siesmic zone.